Endowment Effect

The tendency to value something more highly simply because one owns it.

A homeowner lists their house at a price significantly above market value because they believe it's special, even though comparable homes sell for less. Ownership has inflated the homeowner's perception of the property's value beyond its objective market worth.
Someone refuses to sell a collectible item for $200 that they would never pay $200 to acquire if they didn't already own it. Possession has created value beyond what the person would attribute to the same item if it belonged to someone else.

Note

First identified by Richard Thaler in 1980 and later studied extensively in collaboration with Kahneman and Knetsch.

This is a common bias